Among plastic resins, engineering plastic resins are characterized by excellent shock resistance, heat resistance, electric properties, and transparency and high processing accuracy. However, they often have high melting points and therefore need to be processed at higher temperatures than general-purpose plastic resins.
Pigments used to color engineering plastics are required to have high heat resistance and high color strength. However, at present, not many red pigments have heat resistance high enough for engineering plastics such as nylon. No commercial pigments meet the required characteristics such as heat resistance and color strength, and there is a need for pigments having higher heat resistance.
In an invention proposed in order to improve heat resistance (reference 1), the diameter of primary particles is increased, but transparency and color strength are low. The large particle size is advantageous in terms of heat resistance but causes a problem in coloring performance.
Reducing the size of the primary particles allows an improvement in color strength but makes it difficult to disperse the particles and causes deterioration in heat resistance because of the particle size. It is therefore difficult to achieve color strength and heat resistance simultaneously.